This invention relates to low-pressure mercury vapor fluorescent lamps.
Low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and rare gas to maintain a gas discharge during operation. The radiation emitted by the gas discharge is mostly in the ultraviolet (UV) region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation.
There is an increase in the use of fluorescent lamps because of reduced consumption of electricity. To further reduce electricity consumption, there is a drive to increase efficiency of fluorescent lamps, referred to as luminous efficacy which is a measure of the useful light output in relation to the energy input to the lamp, in lumens per watt (LPW).
Thus, more efficient and longer life fluorescent lamps are desired. However, a significant excess of mercury is introduced into the lamp to meet desired long lamp lifetime of up to 20,000 hours or more. This is necessary because different lamp components, such as the glass envelope, phosphor coatings and electrodes use up the mercury in the lamp. Such increased use of mercury is not desirable and is detrimental to the environment. Accordingly, there is a drive to reduce mercury consumption in fluorescent lamps without a reduction in the lamp life.
An example of a successful lamp with reduced mercury consumption is the Alto Econowatt fluorescent lamp. These lamps use large-particle cool-white calcium halophosphate phosphor having an average particle size of about 12 to 16 microns and are doped with less mercury than other lamps to meet the TCLP requirement for non-hazardous waste. To continue to meet the rated life of these lamps, it is essential that the lamp and its components have low mercury consumption.
Similarly, fluorescent lamps of Daylight/Daylight Deluxe color have used a large-particle blue-halo calcium halophosphate phosphor as part of a two-component blend that uses a standard white phosphor or a warm-white phosphor as the other component. These lamps are doped with less mercury to meet the TCLP requirement for non-hazardous waste.
There is a continued need for fluorescent lamps with reduced mercury that pass the TCLP standards.
An object of the present invention is to provide fluorescent lamps of cool-white color with reduced mercury consumption.
Another object of the invention is to provide phosphor blends that are useful in the manufacture of such fluorescent lamps of cool-white color with reduced mercury consumption.
The present invention accomplishes the above and other objects by providing an electric lamp having an envelope with an inner surface and at least one electrode, preferably electrodes located at both ends of the envelope tube. The lamp may be a straight fluorescent tube, for example of the type as illustrated in the embodiment of the invention shown in FIG. 1 such as T12 straight Econowatt lamps, or it may be a lamp that includes an envelope of convoluted configuration to a desired shape such as an envelope having at least two straight leg segments joined by a U-bent section as illustrated in the embodiment of the invention shown schematically in FIG. 2 or as in PL lamps, Circleline lamps, SLS lamps, etc. In either embodiment, the electrodes transfer electric power to generate ultraviolet radiation in the envelope which is filled with mercury and a charge sustaining gas.
Optionally, as in the case of the straight envelope fluorescent lamps, the inner surface of the envelope may be pre-coated with a metal oxide layer, such as an aluminum oxide layer, to reflect ultraviolet radiation back into the envelope. Such pre-coats are not customarily used in the case of lamps with convoluted envelopes although a flexible pre-coat may be used in the case of SLS lamps as mentioned further hereinbelow.
A phosphor layer is formed over the inner surface, pre-coated or not, to convert the ultraviolet radiation to visible light. In conventional lamps, the phosphor layer for a conventional F34T12 straight Econowatt fluorescent lamp is preferably a large particle-sized cool-white calcium halophosphate phosphor formed from a coating which comprises calcium halophosphate activated with manganese and antimony. Similarly the phosphor layer for a conventional U-bend fluorescent lamp of cool-white color contains a large particle-sized two phosphor mix of about 50% large particle cool-white calcium halophosphate activated with antimony and manganese, and about 50% fines of cool-white calcium halophosphate activated with manganese and antimony. The fines are normally used to achieve good adhesion particularly in the convoluted or bent areas between the glass layer or coatings thereon and the phosphor layer.
We have discovered that the color obtained from the conventional large particle phosphor blend can be achieved by a phosphor derived from a mixture of fines of warm-white calcium halophosphate phosphor, small-particle blue-halo calcium halophosphate phosphor, and calcium-yellow calcium halophosphate phosphor. It has been found further that using this phosphor blend makes it possible to achieve good adhesion in the manufacture of convoluted lamps of the U-bend type while using low mercury doses in the fluorescent lamp making it environmentally benign.
In preferred embodiments of the invention, a cool-white U-bend fluorescent lamp is provided having a phosphor that comprises a mixture of
(1) a blue-halo calcium halophosphate phosphor having an average particle size within the range of about 6.6 to about 10 microns, most preferably having an average particle size of about 8.60 microns in a mixture comprising about 18% of the blue-halo calcium halophosphate phosphor;
(2) a calcium-yellow calcium halophosphate phosphor having an average particle size within the range of about 9.0 to about 13 microns, most preferably having an average particle size of about 11.3 microns in a mixture comprising about 41% of the calcium-yellow calcium halophosphate phosphor; and
(3) fines of a warm-white calcium halophosphate phosphor of randomly occurring particle size, most preferably having an average particle size of about 4.62 microns in a mixture comprising about 41% of the warm-white phosphor.
Such phosphor blends result in low-mercury consuming lamps. permit use of reduced amounts of mercury when compared to commercially available lamps (other than the Philips Alto lamps) produced with the large particle phosphors in which more mercury is required.
Mercury consumption is determined by the quantity of mercury which is bound on lamp components during operation of the lamp and is thus no longer available for operation of the lamp. In the present invention, it is possible to have reduced amounts of mercury doped in fluorescent lamps and preferably in cool-white U-bend fluorescent lamps, making such lamps environmentally benign and TCLP compliant.
Lamps derived from such phosphors of the invention also exhibit excellent long-life characteristics.
While the exact reasons for such observations are not known with certainty and we do not wish to be bound by any particular theory, it is believed that due to the small particle size of the warm-white fines and of the blue-halo phosphor, the phosphor of the invention provides good packing of the grains of the phosphor coating on the lamp and good shielding of the glass providing an improved barrier that reduces mercury loss in glass.
In lamps of the invention, the initial dose of elemental mercury is provided in such a quantity that:
(A) after about 2,500 hours of lamp operation a sufficient quantity of elemental mercury is available to support a column discharge, and
(B) said lamp is TCLP standard compliant.
This is a real advantage, since the lamps pass the TCLP test through actual reduction in the amount of mercury in the lamp.
Thus the invention in preferred embodiments encompasses an electric lamp which comprises:
a lamp envelope having an inner surface;
means within the lamp envelope for generating ultraviolet radiation; and
a layer of a luminescent material that includes a phosphor derived from a mixture of:
(1) a blue-halo phosphor having an average particle size within the range of about 6.6 to about 10 microns, most preferably having an average particle size of about 8.60 microns in a mixture comprising about 18% of the blue-halo phosphor;
(2) a calcium-yellow phosphor having an average particle size within the range of about 9.0 to about 13 microns, most preferably having an average particle size of about 11.3 microns in a mixture comprising about 41% of the calcium-yellow phosphor; and
(3) fines of a warm-white phosphor of randomly occurring particle size, most preferably having an average particle size of about 4.62 microns, in a mixture comprising about 41% of the warm-white phosphor.